The present invention relates to a Cu-alloy mold for use in centrifugal casting of Ti and Ti alloys superior in dimensional accuracy, and to a centrifugal casting method using the mold.
Ti or Ti alloy has widely been used in many fields as Ti or Ti-alloy castings, because of their superior corrosion resistance and specific strength.
Ti or Ti alloys cannot be cast in normal or usual foundry sand molds, because they are chemically active. For this reason, for casting Ti or Ti alloys, used are a graphite mold, a precision casting mold, utilizing specific ceramics, typically represented by a lost-wax process, and the like. Further, a water-cooled Cu mold or the like is also used for casting of the Ti or Ti alloy.
However, the above-described graphite mold is expensive, and lacks dimensional precision. Furthermore, since the precision casting mold represented by the lost-wax process must employ expensive ceramics, use of the precision casting mold increases the cost of production.
On the other hand, the water-cooled mold is a mold which is most suitable for mass production as a general continuous casting mold. Casting of the Ti or Ti alloy by the use of the water-cooled mold is considered to enable Ti or Ti-alloy castings superior in dimensional precision, to be produced. However, the following problems arise. That is, if the water-cooled mold is to be used as a centrifugal casting mold, it is difficult to construct water-cooling mechanisms because the latter is rotated during the centrifugal casting. If molten Ti or molten Ti alloy is cast in the Cu mold without water cooling, the Cu mold is considered to be heated to temperatures in excess of the heat-resistant temperature. For this reason, melting loss or seizure will occur on a surface of a cavity in the Cu mold into which the Ti or Ti alloy is cast. Further, breakage or deformation will occur in the Cu mold body due to thermal stresses. Thus, it is impossible to produce Ti or Ti-alloy castings which are superior in dimensional accuracy.
Apart from the above, where a precision product made of a titanium alloy is to be manufactured, it is general that, since the titanium alloy is high in melting point and is reactive, a mold made of ceramics is used to cast the product.
Where relatively small components, for example, valve heads or the like used in engines are mass produced, of a titanium alloy, the following method has been taken. That is, in the method, a plurality of molds made of ceramics each having a plurality of molding cavities are stacked one upon the other vertically in a manner of a plurality of stages or steps, and the molds are rotated about a vertical central axis (truck) to cause a centrifugal force to act upon molten metal within the cavities, thereby spreading the molten metal to every nook and corner of the cavities (branches), to form the valve heads. By this method, a cast intermediate article is produced in which the plurality of valve heads are molded at forward-end portions of the branches.
The reason why the aforesaid casting method can be carried into practice is that, after casting, the molds made of ceramics are disposable.
The inventors of this application have considered that, in place of the throw away ceramic molds, copper alloy molds usable repeatedly should be used to cast titanium-alloy products.
The copper alloy molds when used as described above, the following problems arise. That is, when the products are to be taken out after casting, even if each of the molds is made as a book mold, the mold halves will interfere with a cast article. Thus, the mold halves cannot be opened sufficiently to take out the products.
Furthermore, generally, an engine valve made of a titanium alloy is light and strong or tough and, accordingly, such valves are employed in high r.p.m. engines for racing cars. Particularly, the engine valve made of the titanium alloy, which is used as an exhaust valve, can be heated to temperatures of the order of 700.degree. C. by exhaust gas. In view of this, as shown in FIG. 4 of the attached drawings, the following method has been proposed. That is, a valve head 100 is made of a titanium alloy which has the following composition:
Al: 5.5 to 6.5 wt %, PA1 Sn: 1.8 to 2.2 wt %, PA1 Zr: 3.6 to 4.4 wt %, PA1 Mo: 1.8 to 2.2 wt %, PA1 Si: equal to or less than 0.10 wt %, and PA1 the remainder: Ti and unavoidable impurities. PA1 Al: 5.5 to 6.75 wt %, PA1 V: 3.5 to 4.5 wt %, PA1 Fe: equal to or less than 0.30 wt %, and PA1 the remainder: Ti and unavoidable impurities. PA1 a Cu--Zr alloy, PA1 a Cu--Cr--Zr alloy, PA1 a Cu--Be alloy, PA1 a Cu--Cr alloy, and PA1 a Cu--Ag alloy. PA1 Ts is the tensile strength (kg/mm.sup.2), and .rho. is the electrical conductance (% IACS), and wherein the cavity has its volume which is at most 30% of the volume of the mold body; PA1 at least one spacer means interposed between the molds, the spacer means being capable of being split into a plurality of spacer sections; and PA1 means for fixing the molds and the spacer means to each other under such a condition that the molds and the spacer means are stacked one upon the other. PA1 Al: 7 to 12 wt %, PA1 Sn: 0.5 to 5 wt %, PA1 Zr: 0.5 to 6 wt %, PA1 Mo: 0.5 to 5 wt %, and PA1 the remainder: Ti and unavoidable impurities; and PA1 Ts is the tensile strength (kg/mm.sup.2), and .rho. is the electrical conductance (% IACS), and the cavity having its volume which is at most 30% of volume of the mold body; and PA1 centrifugally casting the melted Ti alloy in the permanent mold, to form the engine valve head.
The above-described titanium alloy will hereunder be referred to as "Ti-6Al-2Sn-4Zr-2Mo-0.1Si alloy".
On the other hand, a valve stem 200 is made of a titanium alloy which is superior in processability and which has the following composition:
The above-mentioned titanium alloy will hereunder be referred to as "Ti-6Al-4V alloy".
The valve head 100 and the valve stem 200 are joined together by a friction welding process or the like, to manufacture an engine valve 300 made of the titanium alloy.
Since the valve stem 200 is made of the Ti-6Al-4V alloy which is superior in processability, it is possible to easily manufacture the valve stem 200 by plastic processing. Since, however, the valve head 100 is made of the Ti-6Al-2Sn-4Zr-2Mo-0.1Si alloy which is inferior in processability, the valve head 100 has been manufactured such that the alloy is melted in a plasma-arc furnace and, subsequently, is centrifugally cast by the use of a graphite permanent mold.
Without being limited to the racing engines, the engine weight has been reduced in recent years, and the engine speed had tended to increase. In keeping with this, a valve head section of the exhaust valve is heated to a temperature exceeding 800.degree. C. High-temperature strength is insufficient in the conventional valve head made of Ti-6Al-2Sn-4Zr-2Mo-0.1Si-alloy casting. Thus, there is a problem in durability. If the Al content increases in the titanium alloy, the tensile strength and the 0.2% proof stress increase. However, there is a tendency that elongation is reduced. Particularly, it has been known that elongation at normal temperature is excessively reduced.
Accordingly, the exhaust valve, which has the valve head made of the titanium alloy rich in Al content, has no problem in use at high temperature. If, however, the exhaust valve is used at the normal temperature, for example, if the exhaust valve receives a shock like in the start-up of the engine, there may be a case where cracks or breaking loss occur in parts of the valve head, because of its brittleness. Thus, there is a problem that the valve head cannot be put into practical use.